1. Field of the Invention
This invention relates to perpendicular magnetic recording media and to a method of manufacturing the media. The invention relates to high areal density perpendicular magnetic recording media having a magnetic layer exhibiting high corrosion resistance. Furthermore, the invention relates to magnetic materials particularly suitable for forming a magnetic layer exhibiting high corrosion resistance in granular perpendicular recording media applications.
2. Description of the Related Art
Perpendicular recording media are being developed for higher density recording as compared to longitudinal media. The thin-film perpendicular magnetic recording medium comprises a substrate and a magnetic layer having perpendicular magnetic anisotropy, wherein the magnetic layer comprises an easy axis oriented substantially in a direction perpendicular to the plane of the magnetic layer. Typically, the thin-film perpendicular magnetic recording medium comprises a rigid NiP-plated Al alloy substrate, or alternatively a glass or glass-ceramic substrate, and successively sputtered layers. The sputtered layers can include one or more underlayers, one or more magnetic layers, and a protective overcoat. The protective overcoat is typically a carbon overcoat which protects the magnetic layer from corrosion and oxidation and also reduces frictional forces between the disc and a read/write head. In addition, a thin layer of lubricant may be applied to the surface of the protective overcoat to enhance the tribological performance of the head-disc interface by reducing friction and wear of the protective overcoat.
Granular perpendicular recording media is being developed for its capability of further extending the areal recording density as compared to conventional perpendicular recording media which is limited by the existence of strong exchange coupling between magnetic grains. In contrast to conventional perpendicular media wherein the magnetic layer is typically sputtered in the presence of inert gas, most commonly argon (Ar), deposition of a granular perpendicular magnetic layer utilizes a reactive sputtering technique wherein oxygen (O2) is introduced, for example, in a gas mixture of Ar and O2. Not wishing to be bound by theory, it is believed that the introduction of O2 provides a source of oxygen that migrates into the grain boundaries forming oxides within the grain boundaries, and thereby providing a granular perpendicular structure having a reduced exchange coupling between grains. However, the migration of oxygen and the oxidation process produces a granular perpendicular magnetic layer having a porous structure significantly more susceptible to corrosion.
The continuing drive for increased areal recording density in the magnetic recording media industry mandates reduction of the head-to-medium separation, or more particularly the head to magnetic layer separation. As such, an increase in areal density usually requires a reduction in the thicknesses of the intermediary layers, namely the protective overcoat and the lubricant layer, that constitute part of the head to magnetic layer separation. One role of the protective overcoat is to prevent corrosion of the underlying magnetic layer, which is an electrochemical phenomenon dependent upon factors such as environmental conditions, e.g., humidity and temperature. A suitable protective overcoat must prevent migration of ions, such as cobalt (Co) and nickel (Ni), from underlying layers to the surface of the magnetic recording medium which can form defects such as asperities. However, as the protective overcoat thickness is reduced to below 40 Å, the magnetic layer becomes more vulnerable to corrosion. Such low thicknesses reduce the ability of the protective overcoat to maintain adequate corrosion protection.
Accordingly, there exists a need for perpendicular magnetic recording media having a high recording areal density, and a significantly reduced head-to-medium separation while simultaneously providing adequate resistance to environmental attacks, such as corrosion. There exists a particular need for high recording areal density magnetic recording media having a combined protective overcoat and lubricant film thickness less than about 60 Å and exhibiting improved corrosion resistance. There is a need for granular perpendicular recording media having a magnetic layer exhibiting improved corrosion resistance while maintaining the magnetic properties suitable for high density perpendicular recording.